California farmers hope crops get a chance to chill

Just as people benefit from getting the proper amount of sleep, crops like pistachios, peaches and almonds need a certain kind of rest as well.

Typically referred to in the agricultural industry as “chill hours,” it is basically a certain amount of cold weather needed every year — usually between 32°F and 45°F —to help set buds on the tress that will turn into flowers in spring, then into fruits and nuts in summer.

For pistachio trees near Fresno, California, for example, they require more than more than 700 hours of chill time every winter, but for the past four years, many have had less than 500 hours. As a result, these tress do not bloom uniformly, which can dramatically reduce the yield, explained Tom Coleman, a farmer of more than 8,000 acres of pistachio trees across the state.

This is a problem that farmers are facing across California, and if it continues, the prices for these products could go up. Multiple University of California studies predict that within 30 to 50 years it may be too warm to grow many tree crops where they now flourish.

Agricultural scientist Eike Leudeling found that climate conditions in California by “the middle to the end of the 21st century will no longer support some of the main tree crops currently grown.” He explained in a recent article that farmers will either need to find alternative crops or establish ways to mitigate warming temperatures.

UC Davis researcher Hyunok Lee, whose study was published in the journal California Agriculture, found that winter temperatures are increasing more than at any other time of year. Her model looks at the year 2050 in Yolo County.

“Our agriculture will continue,” Lee said in the article. “But if you look at . . . 20 years or 30 years, the pattern may change a little bit, crops may move a little bit north.”

She says tree crops like walnuts would be harmed the most, but annual crops like tomatoes could actually benefit from rising temperatures. For growers with huge investments in trees that have life cycles of 25 years or more, this is a big deal.

Farm adviser David Doll is trying different things to get the trees more sleep. He has experimented with overhead sprinklers and even painting the trees white with liquid clay to reflect sunlight.

“This is something that could impact a lot of farmers over the next 10 to 40 years,” Doll says. “It’s already impacting farmers on random given years across the state.”

California’s pistachio industry has already been hit hard due to a lack of chill hours and in 2015, the crop was nearly split in half. The UC system and the pistachio industry have invested about a million dollars to figure out how to cope with warming temperatures, including trying to breed a pistachio tree that needs less sleep.

“We’re trying to use the other species of pistachios to see if we can come up with something that has a low-chill requirement. It’s pretty hypothetical at this stage,” farm adviser Craig Kallsen said in the article. “We made quite a few crosses this spring and we actually hope to put a trial in a low-chill area.”

And it’s not only pistachios that are suffering. Cherries may suffer this year as well because of a warmer winter. But, the only real solution as of now is for the temperature to drop. To complicate the matter, many researchers and farmers say there isn’t enough understanding about why the trees need sleep. Plus, a decrease in the amount fog in the region also keeps trees from staying cool. That has to do with rain.

In some areas of California, it has recently rained so much that many farmers are hoping the ground will remain saturated enough for fog to form this winter, which would lower the temperature around their trees.

“The fog is a good thing because it keeps a uniform cooler temperature on the ground, but we just haven’t seen that over that last several years — even with the rain,” Coleman said.